<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <script>
        var a;
        console.log(a);
        a=2;
        console.log(a);
        console.info(a);
        console.warn(a);
        console.error(a);
        console.debug(a);
        console.log(123)
    </script>

</body>
</html>